This invention relates to a piezoelectric relay which uses a piezoelectric actuator as a contact driving source and, more particulary, to a piezoelectric relay having a multilayer piezoelectric actuator comprising a plurality of stacked piezoelectric ceramic laminates.
Almost all the relays which have been put into practical use are electromagnetic relays using exciting coils as the contact driving source. Even in the age when semiconductor technology has progressed remarkably, the demand for electromagnetic relays continues to increase in various fields because they endure higher voltage and have better switching characteristics, as compared with semiconductor switching elements. However, those relays which are equipped with the exciting coils cannot avoid having a large power comsumption, high heat generation, and large size fabrication. Also, the relays tend to generate magentic fields which affect adjacent circuit elements, such as those elements which are mounted on a printed wiring board.
Recently, in order to solve these and other problems of electromechanical relays, efforts have been made to develop piezoelectric relays which use a piezoelectric actuator, in place of the exciting coils, as the contact driving source. As is well known, a piezoelectric actuator is a transducer for converting between electrical energy and mechanical energy, the actuator physically deforms to cause a displacement responsive to an application of voltage onto piezoelectric ceramic laminates. The piezoelectric actuators are classified into the piezoelectric transverse effect type or bimorph type whose laminates are bendably displaced due to mechanical strain occurring in the vertical direction in response to an electrical field and the piezoelectric longitudinal effect type or multilayer type whose laminates are expandably displaced due to the mechanical strain occurring in the direction which is parallel to the electrical field.
The relays using the piezoelectric transverse effect type or bimorph type actuators as the contact driving source have been shown in the structures disclosed, for example, in U.S. Pat. Nos. 4,403,166 and 4,425, 524. In these relays, a bimorph type actuator can give a large displacement to a movable contact, but bending displacement which is caused by the expansion of two laminates consumes energy and inveitably lowers the energy conversion efficiency. If the relays are required to be miniaturized to be mounted with other circuit elements on a printed wiring board, they cannot apply sufficient contact pressure between a movable contact and a stationary contact. When the driving voltage is applied continuously to the bimorph type actuator, it becomes impossible to stably open and close the contacts for a long time of period, because there is a change in the displacement charcteristics. Therefore, the piezoelectric relays using the bimorph type actuators have heretofore not been put into practical use.
The relays using a piezoelectric longitudinal effect type actuator as the contact driving source, on the other hand, have a disadvantage because the degree of obtainable displacement is extremely small as compared with the relays using th bimorph type actuator. The disadvantage can be solved by increasing the voltage applied on the actuator to compensate for the electrical field intensity. However, the applicable voltage is necessarily limited within a certain range because the driving control circuit of the actuator has a low voltag resistance. However, such a limitation imposed on the applied voltage becomes a bottleneck problem in the practical use of the relays with this type of the actuators.
U.S. Pat. No. 4,454,442 discloses an example of a piezoelectric relay which attempts to solve the bottleneck problem. The relay disclosed therein has a structure which enables a minute displacement occurring in the longitudinally expandable piezoelectric body (actuator) to be magnified by a single resilient elongated member made of a dielectric material or by a mechanical amplification member so as to make connect between contacts. More particularly, in this type of relay structure, a minute displacement (several .mu.m) of the piezoelectric body should be magnified several tens of times by bending the single resilient member to gain a sufficient distance (e.g. 0.55 mm) for moving the contacts. However, it is almost impossible to magnify the displacement at a high precision, in the case of a miniaturized relay.